Guill was a Mari merchant who dealt in violent thoughts, which were outlawed on his homeworld. He ran the black market trade of these violent images. When USS Voyager spent a few days in orbit of his homeworld, Guill, intrigued by the possibilities offered by the thoughts of Voyager''s more violent crewmembers such as B'Elanna Torres, set up an incident where Torres would be angered long enough for him to acquire some of her more violent thoughts.
Although he briefly started to attack someone when the thought became too much for him, he was able to pass the thought on to others, causing at least one death and framing Torres as the source, resulting in her being scheduled for a potentially brain-damaging engramatic purge.
Tuvok, after a mind meld with Torres, saw the probing done by Guill, and investigated him. Under the pretense of selling Guill more violent thoughts, he mind-melded with him, but instead of transferring violent images, he caused Guill great pain, the sheer intensity of a Vulcan's suppressed darker emotions proving too much for Guill to handle, forcing him to confess. (VOY: "Random Thoughts")